07 Mar 2008

In the drama unfolding within the mysterious protein clumps that crop up in amyotrophic lateral sclerosis (ALS) and other neurodegenerative disorders, a newer player could be joining the current star under the limelight. TAR DNA binding protein (TDP-43) took center stage in late 2006, when it was discovered in the so-called inclusion bodies found in ailing motor neurons of patients with ALS or frontotemporal lobar degeneration with ubiquitin-positive inclusions (FTLD-U) (see ARF related news story). Reporting in last week’s Journal of Biological Chemistry online, scientists have reproduced ALS/FTLD-U phenotypes in cultured cells by introducing TDP-43 mutants incapable of proper cellular trafficking. Another research team, publishing February 20 in the Journal of Neuroscience, has identified a novel splice form of peripherin, a lesser studied component of ALS inclusions, and found it upregulated in ALS and associated with disease pathology. Given TDP-43’s role as a splicing factor, some now wonder whether TDP-43 abnormalities could give rise to the odd peripherin isoforms that might underlie ALS. Of course, many ifs and buts stand in the way of a direct connection between TDP-43 and peripherin, but it is tempting for now to speculate such interplay.

In the first study, researchers led by Virginia Lee—whose group initially identified ubiquitinated TDP-43 in ALS and FTLD-U inclusions—strengthen the case for TDP-43’s starring role. This paper from her team at the University of Pennsylvania School of Medicine, Philadelphia, comes at the heels of a pair of studies that analyzed DNA from ALS patients and found mutations in a region of TDP-43 thought to be important for protein-protein interactions (see ARF related news story).

A predominantly nuclear protein in normal cells, TDP-43 dramatically redistributes into insoluble aggregates in the cytoplasm of diseased motor neurons in ALS and FTLD-U patients. How this occurs remains a mystery, but part of the answer might lie within the signals that direct TDP-43 in and out of the nucleus, the scientists found. By expressing TDP-43 mutants containing faulty nuclear localization or nuclear export sequences, first author Matthew Winton and colleagues were able to recreate in cultured cells the signature redistribution of TDP-43 found in ALS and FTLD-U lesions. The findings suggest that ALS/FTLD-U pathology could be linked to disturbed intracellular trafficking of TDP-43.

Whereas the 2006 discovery of TDP-43 in ALS/FTLD-U lesions has unleashed a flood of some 80 papers since, the peripherin line of investigation to date remains confined to a couple of labs. One such group, led by Janice Robertson at the University of Toronto, Ontario, reported several months ago the co-localization of peripherin with TDP-43 in the same ALS inclusions (Sanelli et al., 2007). In that paper, they also raised the possibility—based on more detailed structural representations of these inclusions obtained from three-dimensional deconvolution imaging—that TDP-43 may not be the key ubiquitinated target within the inclusions.

In the intensifying quest to understand these enigmatic structures, Robertson’s present paper has brought peripherin into closer view. What led her team to suspect aberrant peripherin splicing in ALS in the first place? It has been known for nearly a decade that overexpression of peripherin, a neuronal intermediate filament protein, kills motor nerve cells in transgenic mice (Beaulieu et al., 1999), and variations within the peripherin gene have come to light in a few ALS cases. In addition, Robertson and colleagues have shown abnormal expression of a neurotoxic peripherin splice form in transgenic mouse models of ALS (Robertson et al., 2003). While hunting for the human equivalent of this neurotoxic splice variant, the scientists stumbled upon a novel peripherin isoform, which they describe in the current paper.

Shangxi Xiao and colleagues report that this new variant, dubbed Per28, is expressed at low levels in human spinal cord cells, upregulated in ALS tissue, and associates with round inclusion bodies characteristic of ALS and FTLD-U. The scientists have yet to look for Per28 in brain tissue from FTLD-U patients, Robertson says, but that’s next on the agenda. So far, their analysis of brain sections from people with Alzheimer and Parkinson diseases has revealed no Per28.

Given that regulation of exon splicing is one of TDP-43’s known functions, it is “tempting to speculate that the deregulated splicing we have observed for peripherin may be causally linked to abnormalities of TDP-43,” write the authors.

Noting the loss of TDP-43 immunoreactivity in the nucleus where it would normally function, Robertson told ARF, “we are trying to deplete TDP-43 in the nucleus and determine if this affects peripherin splicing.”—Esther Landhuis

Esther Landhuis is a science journalist in Dublin, California.

Comments

1. • Jean-Pierre Julien
     
   • Posted: 21 Mar 2008
   • Paper: An aggregate-inducing peripherin isoform generated through intron retention is upregulated in amyotrophic lateral sclerosis and associated with disease pathology.

   Q&A with Jean-Pierre Julien. Questions by Esther Landhuis.

   Q: It seems that the idea that intermediate filaments are involved in ALS has gotten little attention. Why do you think this is the case?

   A: Previous transgenic mouse studies showed that overexpression of peripherin can provoke progressive motor neuron degeneration. The idea of intermediate filaments (IF) in ALS got some attention because of transgenic mouse studies showing that IF abnormalities can cause motor neuron disease. The concept that IFs might be involved in ALS is also appealing because IFs are abundant in large motor axons, and it would make sense that motor neurons might be vulnerable to cytoskeletal disorganization. However, the genetic screening has been disappointing. To date, only a few genetic variants in neurofilament NFH and peripherin genes have been reported in ALS patients, and it is unclear that these mutations are causative.

   Q: What is your perspective on this paper?

   A: It demonstrates that abnormal peripherin gene splicing does occur in sporadic ALS, and that the truncated P28 peripherin resulting from intron 3 and 4 retention can form aggregates. This is a very nice study. It is striking that the P28 protein is detected in ALS but not in controls. However, it remains unknown at this time to what extent the presence of this P28 protein might contribute to motor neuron degeneration in ALS. The results in Figure 5 of the paper indicate that this protein did not exhibit a high level of toxicity when expressed in cultured motor neurons. More work is needed to address this issue.

   View all comments by Jean-Pierre Julien

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References

News Citations

1. New Ubiquitinated Inclusion Body Protein Identified 8 Oct 2006
2. Gene Mutations Place TDP-43 on Front Burner of ALS Research 29 Feb 2008

Paper Citations

1. Sanelli T, Xiao S, Horne P, Bilbao J, Zinman L, Robertson J. Evidence that TDP-43 is not the major ubiquitinated target within the pathological inclusions of amyotrophic lateral sclerosis. J Neuropathol Exp Neurol. 2007 Dec;66(12):1147-53. PubMed.
2. Beaulieu JM, Nguyen MD, Julien JP. Late onset of motor neurons in mice overexpressing wild-type peripherin. J Cell Biol. 1999 Nov 1;147(3):531-44. PubMed.
3. Robertson J, Doroudchi MM, Nguyen MD, Durham HD, Strong MJ, Shaw G, Julien JP, Mushynski WE. A neurotoxic peripherin splice variant in a mouse model of ALS. J Cell Biol. 2003 Mar 17;160(6):939-49. PubMed.

Further Reading

News

1. New Ubiquitinated Inclusion Body Protein Identified 8 Oct 2006
2. Gene Mutations Place TDP-43 on Front Burner of ALS Research 29 Feb 2008